Double metal cyanide (DMC) compounds are well known catalysts for epoxide polymerization. The catalysts are highly active, and give polyether polyols that have low unsaturation compared with similar polyols made using basic (KOH) catalysis. Conventional DMC catalysts are prepared by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. The catalysts can be used to make a variety of polymer products, including polyether, polyester, and polyetherester polyols. Many of the polyols are useful in various polyurethane coatings, elastomers, sealants, foams, and adhesives.
DMC catalysts are usually prepared in the presence of a low molecular weight organic complexing agent, typically an ether such as glyme (dimethoxyethane) or diglyme. The complexing agent favorably impacts the activity of the catalyst for epoxide polymerization. Other known complexing agents include alcohols, ketones, esters, amides, ureas, and the like. Recently, we described substantially amorphous DMC catalysts prepared using water-soluble aliphatic alcohol complexing agents such as tert-butyl alcohol in U.S. Pat. No. 5,470,813.
In one conventional preparation, aqueous solutions of zinc chloride and potassium hexacyanocobaltate are combined. The resulting precipitate of zinc hexaoyanocobaltate is combined with an organic complexing agent. The resulting catalyst has the general formula: EQU Zn.sub.3 [Co(CN).sub.6 ].sub.2 .multidot.xZnCl.sub.2 .multidot.yH.sub.2 O.multidot.zComplexing agent
DMC catalysts are made with an excess of the metal salt compared with the amount of metal cyanide salt used. See, e.g., U.S. Pat. Nos. 3,427,256, 3,278,457, and 3,941,849. More recently, we taught (U.S. Pat. No. 5,158,922) an improved process for making easily filtered DMC catalysts by controlling the order of reagent addition, the reaction temperature, and the stoichiometric ratio of the reactants. The '922 patent teaches to use at least about a 100% stoichiometric excess of the metal salt relative to the metal cyanide salt. Thus, in the example above, at least about 3 moles of zinc chloride is used per mole of potassium hexacyanocobaltate. The examples in the reference use glyme as the organic complexing agent. Zinc hexacyanocobaltate catalysts prepared by this procedure generally have zinc chloride to zinc hexacyanocobaltate mole ratios of about 0.6 or more. The '922 patent discloses (in a formula) compositions having as little as 0.2 moles of metal salt per mole of DMC compound in the catalyst.
While the procedure described in the '922 patent (large excess of zinc chloride) works well with glyme, it is less satisfactory for use with other complexing agents, including tert-butyl alcohol. When tert-butyl alcohol is used, the catalyst precipitate becomes gelatinous and difficult to isolate. In addition, the activity of these catalysts for epoxide polymerizations, although quite high compared with KOH catalysts, is still somewhat less than desirable. The catalysts prepared by the reference procedure with glyme as the organic complexing agent typically polymerize propylene oxide with an activity less than about 2 g PO/min at 100 ppm of catalyst, based on the weight of finished polyol, at 105.degree. C.
Recently, we described substantially amorphous DMC catalysts in U.S. Pat. No. 5,470,813. These catalysts are preferably made using a water-soluble aliphatic alcohol complexing agent such as tert-butyl alcohol. An excess amount of metal salt is used to make the catalyst. Zinc hexacyanocobaltate catalysts described therein have more than 0.2 moles of metal salt per mole of zinc hexacyanocobaltate present, typically more than 0.5 moles of metal salt per mole of zinc hexacyanocobaltate. The X-ray diffraction patterns show that the catalysts are substantially amorphous; i.e., the catalysts are characterized by the substantial absence of sharp lines in the powder X-ray diffraction pattern (see FIG. 5). The catalysts described in the '534 application have far greater activity for polymerizing propylene oxide than previously known catalysts. For example, rates in excess of about 3 g PO/min at 100 ppm of catalyst were achieved.
Improved double metal cyanide catalysts are needed. Preferred catalysts would be easy to prepare and isolate, and would have excellent activity for polymerizing epoxides. Preferred catalysts would give polyether polyols having narrow molecular weight distributions and low unsaturation.